Dichroic azo dyestuffs and liquid crystal composition containing said dyestuffs

ABSTRACT

Provided are novel dichroic azo dyestuffs, which contain skeletons represented by the following general formula: ##STR1## at one terminals of the long axes of the molecules of the azo dyestuffs, and liquid crystal compositions containing the dichroic azo dyestuffs in forms dissolved in liquid crystals. These dichroic azo dyestuffs have large dichroic ratios and high durability, and their compositions are employed as useful display devices.

DESCRIPTION

1. Technical Field

This invention relates to novel dichroic dyestuffs suitable for use inliquid crystals and liquid crystal compositions containing the abovedyestuffs as well as display devices making use of these compositions.

2. Background Art

Liquid crystal display devices have found wide-spread commercial utilityin recent years from the viewpoints of energy saving and size reduction.Most of liquid crystal display devices, which are currently in use,utilize electrooptical effects of twisted nematic liquid crystals andperform display operations with combined use of two polarizing films asessential conditions. Under the circumstances, numerous limitations arehowever imposed upon their applications. As a possible alternativeliquid crystal display system, investigations have been made onso-called guest-host type liquid crystal displays each of which makesuse of electrooptical effects of a colored liquid crystal compositionobtained by dissolving a dichroic dyestuff in a nematic liquid crystal.Some of such liquid crystal displays have already been used as displaydevices in watches, clocks, electrical home appliances, inductrialmeters, etc.

The principle of the guest-host type liquid crystal display systemrelies upon the phenomenon that molecules of a dichroic dyestuff as aguest tend to orient in accordance with the arrangement of molecules ofa liquid crystal as a host. Namely, upon application of an externalstimulus which is usually an electric field, the molecules of the liquidcrystal change the direction of their orientation from the "off" stateto the "on state" and at the same time, the molecules of the dichroicdyestuff vary the direction of their orientation. As a result, thedegree of absorption of light by the molecules of the dyestuff in theformer state becomes different from that in the latter state so that adisplay is performed. The guest-host type liquid crystal display systemis based on this principle.

The dichroic dyestuff useful here is basically required to satisfy thefollowing requirements: (1) to have sufficient color strength even at alow concentration; (2) to have a large dichroic ratio and to exhibitsignificant contrast by application and removal of a voltage; (3) tohave sufficient solubility to the associated liquid crystal; (4) to haveexcellent durability and stability and to be free from deteriorating theperformance of the device even when used over a long period of time;etc.

Various dichroic dyestuffs have been proposed as materials which fulfillthe above-described requirements. Some of such dichroic dyestuffs havealready been used in digital clocks, meters and so on. Under thecircumstances, many of them are however still accompanied by drawbackswhich should still be improved, that is, those having large dichroicratios have poor durability or those excellent in durability are notequipped with such a dichroic ratio as enabling vivid display ofpractical utility.

DISCLOSURE OF THE INVENTION

With the foregoing in view, the present inventors have carried out anextensive investigation. As a result, novel dichroic dyestuffs havinglarge dichroic ratios and excellent durability have now been found.

Namely, the present invention provides a novel dichroic azo dyestuffrepresented by the following general formula (I): ##STR2## wherein Ameans --CN, --COOR₁, ##STR3## R₁ is an alkyl group, alkoxyalkyl group,or a phenyl or cyclohexyl group which may be substituted with an alkylor alkoxy group at the p-position thereof, R₂ and R₃ are individually ahydrogen atom, alkyl, cycloalkyl, alkoxy, alkoxyalkoxy,cycloalkyloxyalkoxy, aryloxyalkoxy or aryl group, halogen atom, or cyanoor dialkylamino group; Y₁ and Y₂ denote individually a hydrogen orhalogen atom or an alkyl, alkoxy, cyano or hydroxyl group, Ar₁ and Ar₂stand individually for ##STR4## W₁ and W₂ are individually an --N═N-- or--N═CH-- group but W₁ is an --N═N-- group when W₂ is an --N═CH-- group,m stands for an integer of 0 or 1 but m is 0 when W₁ is an --N═CH--group; and B means a hydroxyl, alkoxy, alkoxyalkoxy or p-alkylbenzyloxygroup, ##STR5## but B may be a hydrogen or halogen atom, cyano group,##STR6## when W₁ or W₂ is an --N═CH-- group, R₄ is an alkyl group or aphenyl or cyclohexyl group which may be substituted with an alkyl oralkoxy group at the p-position thereof, R₅ means a hydrogen atom oralkyl group, R₆ denotes an alkyl group or ##STR7## R₇ and R₈ meanindividually a hydrogen or halogen atom, hydroxyl, cyano, alkyl,dialkylamino, alkoxy or alkoxyalkoxy group, ##STR8## and R₉ is ahydrogen atom or alkyl group.

The characteristic feature of the dyestuff of this invention resides inthe introduction of a skeleton represented by the following generalformula (II) at one end of the longer axis of the molecule of thecorresponding azo compound: ##STR9## wherein A, Y₁, Y₂ and W₁ have thesame meaning as defined in the general formula (I), whereby thedichroism has been improved significantly in comparison with similarknown azo dyestuffs. More specifically, the dichroic azo dyestuff ofthis invention features the inclusion of the group represented by A atthe terminal of the biphenyl group as shown by the general formula (II).It is therefore clearly distinguished from any known azo dyestuffs.

BEST MODE FOR CARRYING OUT THE INVENTION

Dichroic azo dyestuffs of this invention, which are represented by thegeneral formula (I), can be synthesized by processes such as those to bedescribed below.

For example, in the case of a compound represented by the generalformula (I) in which m is 1 and W₁ and W₂ are both --N═N-- groups, anamine represented by the following general formula (III): ##STR10##wherein A has the same meaning as in the general formula (I) isdiazotized in accordance with a usual method, followed by its couplingwith a compound represented by the following general formula (IV):##STR11## wherein Y₁ and Y₂ have the same meaning as in the generalformula (I) to obtain a monoazo dyestuff represented by the followinggeneral formula (V): ##STR12## wherein A, Y₁ and Y₂ have the samemeaning as in the general formula (I). Thereafter, the monoazo compoundis similarly diazotized in a usual manner, followed by its coupling witha compound represented by the following general formula (VI):

    Ar.sub.1 --NH.sub.2                                        (VI)

wherein Ar₁ has the same meaning as in the general formula (I), therebyobtaining a disazo dyestuff represented by the following general formula(VII): ##STR13## wherein A, Y₁, Y₂ and Ar₁ have the same meaning as inthe general formula (I). This disazo dyestuff is further diazotized by ausual method, followed by its coupling with a compound represented bythe following general formula (VIII):

    Ar.sub.2 --B                                               (VIII)

or by the following general formula (IX):

    Ar.sub.2 --OH                                              (IX)

wherein Ar₂ and B have the same meaning as in the general formula (I) toobtain a compound represented by the general formula (I) or a compoundrepresented by the following general formula (X): ##STR14## wherein A,Y₁, Y₂, Ar₁ and Ar₂ have the same meaning as in the general formula (I).The latter compound is a precursor for the former compound.

The compound (X) is then subjected, together with a halogen compoundrepresented by the following general formula (XI):

    Hal--B'                                                    (XI)

wherein Hal means a halogen atom represented by Cl or Br and B' denotesa precursor group capable of undergoing condensation with the compoundof the general formula (X) to become the same group as B in the generalformula (I), to a dehydrohalogenating and condensing reaction by a usualmethod, thereby obtaining the intended dichroic azo dyestuff of thisinvention.

When m=0, the compound of the general formula (V) is diazotized,followed by its coupling with the compound represented by the compoundrepresented by the general formula (VIII) or (IX) and if necessary,followed further by its condensation with the halogen compoundrepresented by the general formula (IX), thereby obtaining the intendeddichroic compound of this invention in the same manner.

On the other hand, in the case of a compound represented by the generalformula (I) in which W₁ or W₂ is an --N═CH-- group, an amino compoundrepresented by the general formula (V) or (VII) and an aldehyde compoundrepresented by the following general formula (XII):

    OHC--Ar.sub.2 --B                                          (XII)

wherein Ar₂ and B have the same meaning as in the general formula (I)are subjected to dehydrating condensation by a usual method, therebyobtaining a dichroic azo dyestuff of this invention.

Although the dichroic azo dyestuffs of this invention can be synthesizedby such processes as those mentioned above, it is also possible tosynthesize the dichroic azo dyestuffs of this invention by combiningother known unit reactions in various ways.

The thus-obtained crude dyestuffs of the general formula (I) are thenpurified by recrystallization, column chromatography or the like, sothat high-purity dyestuffs can be obtained.

Certain specific representative examples of the dichroic dyestuffs ofthis invention will be described in Examples.

The dichroic azo dyestuffs are however not limited to them. As R₁, R₂,R₃, Y₁, Y₂, Ar₁, Ar₂ and B contained in the general formula (I), thefollowing specific groups may be mentioned.

As specific examples of R₁ in the general formula (I), there may bementioned groups having structural formulae such as: --CH₃, --C₂ H₅,--(CH₂)₃ CH₃, --(CH₂)₇ CH₃, --C₂ H₄ CH(CH₃)CH₂ --(CH₃)₃, --(CH₂)₁₀ CH₃,##STR15## --C₂ H₄ OC₂ H₅, --C₂ H₄ O(CH₂)₃ CH₃. Among these, alkyl groupshaving 15 or less carbon atoms are preferred as R₁ for their dichroiceffects and easiness in preparation.

As specific examples of R₂ and R₃, may be mentioned hydrogen atom, alkylgroups such as --CH₃, --C₂ H₅, --(CH₂)₂ CH₃, --CH(CH₃)₂, --(CH₂)₇ CH₃,--CH₂ C(CH₃)₃ and --(CH₂)₁₀ CH₃, alkoxy groups such as --OCH₃, --OC₂ H₅,--O(CH₂)₇ CH₃, --OC₂ H₄ CH(CH₃)CH₂ C(CH₃)₃ and --O(CH₂)₁₂ CH₃,cycloalkyl groups such as ##STR16## alkoxyalkoxy groups such as --OCH₂CH₂ OC₂ H₅, --OCH₂ CH₂ OCH₂ CH(CH₃)₂, cycloalkyloxyalkoxy groups such as##STR17## aryloxyalkoxy groups such as ##STR18## aryl groups such as##STR19## halogen atoms such as --F, --Cl and --Br, cyano group, anddialkyl amino groups such as ##STR20## Even when R₂ and R₃ are hydrogenatoms, i.e., even in the case of an unsubstituted benzooxazole ring orunsubstituted benzothiazole ring, practically sufficient dichroism isstill exhibited. It is hence advantageous from the standpoint of productcost that R₂ and R₃ are both hydrogen atoms.

As specific examples of Y₁ and Y₂, may be mentioned --H, --Cl, --Br,--I, --F, --CN, --OH, --OCH₃ and --CH₃. Large dichroic effects areexhibited and the purification of reaction products is easy, especially,when Y₁ and Y₂ are both hydrogen atoms. Therefore, Y₁ and Y₂ arepreferably hydrogen atoms at the same time.

On the other hand, specific examples of Ar₁ and Ar₂ may include groupshaving structural formulae such as ##STR21##

As specific examples of B, there may be hydroxyl group, alkoxy groupssuch as --OCH₃, --OC₂ H₅, --OC₄ H₉ (n), --OC(CH₃)₃, --O(CH₂)₅ CH₃,--O(CH₂)₇ CH₃, --OC₂ H₄ CH(CH₃)CH₂ C(CH₃)₃ and --O(CH₂)₁₂ CH₃,alkoxyalkoxy groups such as --OCH₂ CH₂ OC₂ H₅ and --OCH₂ CH₂ OCH₂CH(CH₃)₂ p-alkylbenzyloxy groups such as ##STR22## and dialkylaminogroups such as --N(CH₃)₂ and --N(C₂ H₅)₂.

When B is represented by ##STR23## on the other hand, specific examplesof R₄ may include groups having structural formulae such as --CH₃, --C₂H₅, --(CH₂)₃ CH₃, --(CH₂)₇ CH₃, --C₂ H₄ CH(CH₃)CH₂ --(CH₃)₃, --(CH₂)₁₀CH₃, ##STR24##

When B is represented by ##STR25## hydrogen atom, and alkyl groups suchas --CH₃, --C₂ H₅ and --C₄ H₉ may be mentioned as specific examples ofR₅. As specific examples of R₆, there may be mentioned alkyl groups suchas --CH₃, --C₂ H₅, --C₄ H₉ (n) and --CH₂ CH(CH₃)₂, ##STR26## andnucleus-substituted benzyl groups such ##STR27##

The dichroic azo dyestuffs of the present invention can be incorporatedin liquid crystals for use as color display liquid crystal compositionsin conventional display devices.

Conventional nematic liquid crystals, cholesteric liquid crystals,smectic liquid crystals and chiral nematic liquid crystals can all beused as liquid crystal materials. As their examples, may be mentionedliquid crystals of the biphenyl, terphenyl, phenylcyclohexane,cyclohexylcyclohexane, phenylcyclohexanoate, phenylbenzoate,cyclohexylcyclohexanoate, biphenylcyclohexanoate,phenoxycarbonylphenylbenzoate, Schiff base, phenylbiphenylcarboxylate,pyrimidine, dioxane, cyclohexylmethyl ether and cinnamonitrile types.

The dyestuffs according to the present invention may be used singly.Alternatively, two or more of the dyestuffs may also be used incombination. Any dyestuff concentrations may be employed so long as theydo not exceed the upper limit of solubility of each dyestuff in a liquidcrystal and at the same time, fall within a range in which the moleculesof the dyestuff are sufficiently controlled in orientation by theorientation of the molecules of the liquid crystal. In general, it isrecommendable to use it at a concentration of 0.01-10 wt.%, preferably,0.01-5 wt.% based on the associated liquid crystal.

It is also feasible to mix a dichroic dyestuff of this invention withanother dichroic or non-dichroic dyestuff to use the resultant mixtureas a dichroic dyestuff of a desired color hue. No limitation is imposedin this concern. The dissolution of a dichroic dyestuff in a liquidcrystal is effected by adding a predetermined amount of the dyestuff tothe liquid crystal and stirring the resultant mixture for a long periodof time or heating the resultant mixture to at least a temperature,above which the liquid crystal is converted into an isotropic liquid,and then stirring it at the same temperature. In this manner, thedichroic dyestuff can be formulated into a desired color-display liquidcrystal composition.

Exemplary syntheses of certain azo-type dichroic dyestuffs of thisinvention as well as the structures of some representative dyestuffs andtheir dichroic ratios and dissolved colors in a liquid crystals("ZLI-1565", "ZLI-1840" or "E-8" of Merck & Co., Inc.) will hereinafterbe described in the following Examples.

Dichroic ratios, which will be given in the Examples, are characteristicvalues which demonstrate the utility of the novel dichroic dyestuffs ofthis invention most clearly. The dichroic ratios shown in the Exampleswere each obtained by dissolving each dichroic dyestuff at aconcentration of 1.0 wt.% in a liquid crystal produced by Merck & Co.,Inc (trade name: ZLI-1565, ZLI-1840 or E-8) which is a typical nematicliquid crystal, sealing it within glass liquid crystal cell having a gapof 10 μm which had beforehand been treated so as to ensure homogeneousorientation, placing the resultant liquid crystal cell in the light pathof a spectrophotometer, applying linearly polarized light parallel tothe alignment of the liquid crystal to measure the absorbance (A₁₁)linearly polarized light perpendicular to the arrangement of the liquidcrystal to measure the absorbance (A.sub.⊥), and then making acalculation in accordance with the following equation: ##EQU1## All ofthe dichroic dyestuffs of this invention show extremely high dichroicratios.

EXAMPLE 1

After dissolving 15 parts of a compound represented by the followingformula: ##STR28## in 300 parts of N,N-dimethylformamide (hereinafterdescribed as "DMF"), 69 parts of 35% hydrochloric acid were added. Theresultant mixture was cooled below 5° C., followed by a dropwiseaddition of 22 parts of a 30% aqueous solution of sodium nitrite. Afterstirring the mixture at the same temperature for 2 hours, 3.6 parts ofsulfamic acid were added to obtain a diazotized liquid mixture. Thediazotized liquid mixture was poured in 500 parts of ice water, to whicha solution of 5.5 parts of α-naphthylamine in 30 parts of DMF was addeddropwise, followed by its stirring at 0°-5° C. for 2 hours. Subsequentto adjustment to pH=4 with a 45% aqueous solution of caustic soda, themixture was filtered and the solid matter was washed and then dried toobtain 23.5 parts of a crude product, which was a monoazo dyestuffrepresented by the following formula: ##STR29##

Dissolved in 200 parts of DMF were 4.0 parts of the above-obtainedmonoazo dye, followed by an addition of 10 parts of 35% hydrochloricacid. After cooling the resultant mixture below 5° C., 10 parts of a 10%aqueous solution of sodium nitrite were added dropwise and the mixturewas stirred for 2 hours at the same temperature. Upon addition of 1.0part of sulfamic acid, a diazotized liquid mixture was obtained. Theresultant diazotized liquid mixture was added with 300 parts of 50%acetone water to dissolve the diazotized liquid mixture completely. Tothe resultant solution, a solution of 3.1 parts ofN-(p-ethoxybenzyl)-α-naphthylamine dissolved in 20 parts of DMF wasadded dropwise. After stirring the resultant mixture at 0°-5° C. andpH=3-4 for 3 hours, the mixture was poured with stirring in 1,000 partsof ice water. The resultant mixture was neutralized and filtered. Thesolid matter was washed with water and then dried, thereby obtaining 6.3parts of a crude product which was a disazo dyestuff represented by thefollowing formula: ##STR30## The crude product was dissolved in tolueneand then isolated and purified on a column chromatograph filled withsilica gel powder using toluene as an eluent, thereby obtaining 4.8parts of the above product in its pure and disazo-colored form.

In a small beaker, 1.0 wt.% of the above disazo dyestuff was then addedto the liquid crystal "ZLI-1565" produced by Merck & Co., Inc. Theresultant mixture was heated to about 80° C. to form a completely clearsolution.

After allowing the solution to stand and to cool down, the resultantcolored liquid crystal solution was sealed in a test cell for a liquidcrystal display.

The display cell showed a bluish purple color while no voltage wasapplied. When a voltage of 5 V was applied, it became substantiallycolorless only at its electrodes and very good contrast was produced. Inaddition, the dichroic ratio determined in accordance with theabove-described polarizing light method was 17.7 at its maximumabsorption wavelength of 576 nm.

EXAMPLE 2

The compound represented by the following formula: ##STR31## which wassynthesized in Example 1, was diazotized similarly, followed by itscoupling with N-(p-ethylcarbonyloxybenzyl)-N-methylaniline to obtain adisazo dyestuff represented by the following formula: ##STR32##

The dichroic ratio of the purified dyestuff was 15.3 at its maximumabsorption wavelength of 545 nm.

EXAMPLE 3

Similar to Example 1, a compound of the following formula: ##STR33## wassynthesized first of all. It was then diazotized, followed by itscoupling with a compound represented by the following formula: ##STR34##to obtain a disazo dyestuff represented by the following formula:##STR35##

The dichroic ratio of the purified dyestuff was 16.8 at its maximumabsorption wavelength of 575 nm.

EXAMPLE 4

After dissolving 106 parts of a compound represented by the followingformula: ##STR36## in 1,000 parts of N,N-dimethylformamide, 120 parts of35% hydrochloric acid were added. The resultant mixture was cooled below5° C., followed by a dropwise addition of 125 parts of a 20% aqueoussolution of sodium nitrite. After stirring the mixture at the sametemperature for 2 hours, sulfamic acid was added to obtain a diazotizedliquid mixture. After adjusting the diazotized liquid mixture to pH=4, aDMF solution with 36 parts of α-naphthylamine dissolved therein wasadded dropwise and the resultant mixture was stirred at 0°-5° C. for 3hours. The reaction mixture was poured in 1,500 parts of water, followedby its filtration. The thus-obtained solid matter was washed with waterand then dried, thereby obtaining 116 parts of a crude product which wasa disazo dyestuff represented by the following formula: ##STR37##

Six parts of the above-obtained disazo dyestuff were dissolved in 150parts of DMF, followed by an addition of 10 parts of 35% hydrochloricacid. The resultant mixture was cooled below 5° C. While vigorouslymixing the thus-cooled mixture, 10 parts of a 10% aqueous solution ofsodium nitrite were added dropwise. After stirring the mixture at thesame temperature for 3 hours, sulfamic acid was added to obtain adiazotized liquid mixture. To the diazotized liquid mixture, a solutionof 3 parts of N-(p-ethoxybenzyl)-α-naphthylamine dissolved in 30 partsof DMF was added dropwise and the resultant mixture was then stirred at3°-8° C. for 3 hours. The reaction mixture was poured in 600 parts ofwater. After adjustment to pH=6-7, the mixture was filtered and theresultant solid matter was washed with water and then dried. It wasthereafter dispersed in methanol and stirred for 1 hour. The methanoldispersion was filtered and the resulting solid matter was dried toobtain 7.2 parts of a solid product, which was a trisazo dyestuffrepresented by the following formula: ##STR38## The crude product wasdissolved in toluene and was then isolated and purified by columnchromatography on a column filled with silica gel, thereby obtaining 28parts of the trisazo dye.

Then, 1.0 wt.% of the above trisazo dyestuff was added to the liquidcrystal "ZLI-1565" produced by Merck & Co., Inc. The resultant mixturewas heated to about 80° C. to form a completely clear solution. Afterallowing the solution to stand and to cool down, the resultant coloredliquid crystal solution was sealed in a liquid crystal display device.

The display device showed a bluish purple color while no voltage wasapplied. When a voltage was applied, it became colorless at itselectrodes only and good contrast was produced. In this case, thedichroic ratio was 18.7 at its maximum absorption wavelength of 572 nm.

EXAMPLE 5

Similar to Example 4, the compound of the following formula: ##STR39##was diazotized, followed by its coupling with an amine represented bythe following formula: ##STR40## to obtain a disazo dyestuff representedby the following formula: ##STR41##

The dichroic ratio of the purified dyestuff in "ZLI-1565" was 18.2 atits maximum absorption wavelength of 575 nm.

EXAMPLE 6

Similar to Example 4, after obtaining a disazo dyestuff represented bythe following formula: ##STR42## the compound was diazotized and thencoupled with a compound of the following formula: ##STR43## to obtain atrisazo dyestuff represented by the following formula: ##STR44##

The dichroic ratio of the purified dyestuff in "ZLI-1565" was 18.1 atits maximum absorption wavelength of 582 nm.

EXAMPLE 7

Similar to Example 5, after diazotization of a compound represented bythe following formula: ##STR45## a compound represented by the followingformula: ##STR46## to obtain a disazo dyestuff represented by thefollowing formula: ##STR47##

The dichroic ratio of the purified dyestuff in "ZLI-1565" was 16.4 atits maximum absorption wavelength of 539 nm.

EXAMPLE 8

Monoazo Step:

After dissolving 7 parts of a compound represented by the followingformula: ##STR48## in 400 parts of DMF, 20 parts of 35% hydrochloricacid were added. The resultant mixture was cooled below 5° C., followedby a dropwise addition of 25 parts of a 10% solution of sodium nitrite.After stirring the thus-obtained mixture at the same temperature for 2hours, sulfamic acid was added to obtain a diazotized liquid mixture.Subsequent to adjustment of the diazotized liquid mixture of pH=4,57.5parts of an aqueous solution containing 7.5 parts of sodiumα-naphthylaminemethanesulfonate dissolved therein. The resultant mixturewas stirred at 0°-5° C. for 2 hours. The reaction mixture was thenpoured in 1,000 parts of ice water, followed by its filtration. Thethus-obtained cake was dispersed in 1 l of water. The dispersion wasstirred at 85°-90° C. for 8 hours. It was then filtered, and theresulting solid matter was washed with water and then dried to obtain 8parts of a crude product which was a monoazo compound represented by thefollowing formula: ##STR49##

Disazo step:

After dissolving 4 parts of the above-obtained monoazo compound in 200parts of DMF, 12 parts of 35% hydrochloric acid were added. Theresulting mixture was cooled below 5° C., followed by a dropwiseaddition of 12 parts of a 10% aqueous solution of sodium nitrite. Afterstirring the thus-obtained mixture at the same temperature for 2 hours,sulfamic acid was added to obtain a diazotized liquid mixture. Asolution of 2.2 parts of α-naphthylamine dissolved in 10 parts of DMFwas added dropwise and the resultant mixture was then stirred at 0°-5°C. for 2 hours. The reaction mixture was poured in 500 parts of icewater, followed by its filtration. The resulting solid matter was washedwith water and then dried, thereby obtaining 5 parts of crude productwhich was a disazo compound represented by the following formula:##STR50##

Trisazo step:

Five parts of the above-obtained disazo compound were dissolved in 300parts of DMF, followed by an addition of 10 parts of 35% hydrochloricacid. The resultant mixture was cooled to 0°-5° C., to which 8 parts ofa 10% aqueous solution of sodium nitrite were added dropwise. Afterstirring the resultant mixture at the same temperature for 2 hours,sulfamic acid was added to obtain a diazotized liquid mixture. Afteradding, to 500 parts of ice water, a solution of 2 parts of phenoldissolved in 30 parts of a 10% aqueous solution of caustic soda, thediazotized liquid mixture was added dropwise. The resulting mixture wasstirred at 0°-5° C. and pH=7-9 for 2 hours, followed by its filtration.The resulting solid matter was washed with water and then dried, therebyobtaining 5.2 parts of a solid product which was a trisazo compoundrepresented by the following formula: ##STR51##

The above crude product was dissolved in toluene and then isolated andpurified on a column chromatograph filled with silica gel.

Measurement of dichroic ratios:

Placed in a small beaker were 10 parts of the liquid crystal "ZLI-1840"produced by Merck & Co., Inc. and 0.05 parts of the purified dyestuff.The resultant mixture was heated to about 80° C. to form a completelyclear solution. After allowing the solution to stand and to cool down,the resultant colored liquid crystal solution was sealed in a liquidcrystal display device.

The display device showed a purple color while no voltage was applied.When a voltage was applied, it became substantially colorless only atits electrodes and very good contrast was produced. In addition, itsdichroic ratio was 12.8 at its maximum absorption wavelength of 575 nm.

EXAMPLE 9

Three parts of the compound of the following formula: ##STR52## whichhad been synthesized in the same manner as in Example 8, were dissolvedin 60 parts of DMF, followed by addition of 0.8 parts of potassiumcarbonate and 0.2 part of potassium iodide. After stirring the resultantmixture at 70° C. for 1 hour, 1.5 parts of n-butyl bromide were added,followed by its stirring at 80° C. for 2 hours.

After cooling it down to room temperature, it was poured in 300 parts ofwater. The resultant mixture was filtered, and the resulting solidmatter was washed with methanol and then with water to obtain a crudeproduct which was a compound of the following formula: ##STR53##

The dichroic ratio of the above-purified dyestuff in "ZLI-1840" was 14.2at its maximum absorption wavelength of 560 nm.

EXAMPLE 10

Synthesis and purification were conducted in exactly the same manner asin Example 8 except that anilinemethanesulfonic acid was reacted insteadof α-naphthylamine in the disazo step of Example 8, thereby obtaining atrisazo dyestuff of the following formula: ##STR54##

The dichroic ratio of the purified dyestuff was 13.0 at its maximumabsorption wavelength of 469 nm.

EXAMPLE 11

After dissolving 4.5 parts of a disazo compound represented by thefollowing formula: ##STR55## in 100 parts of DMF, 5 parts of 35%hydrochloric acid were added. The resultant mixture was stirred at roomtemperature for 30 minutes and then cooled to 0°-5° C., followed furtherby a dropwise addition of 8.3 parts of a 10% solution of sodium nitrite.After stirring the thus-obtained mixture at the same temperature for 2hours, sulfamic acid was added to obtain a diazotized liquid mixture.The diazotized liquid mixture was added with 1.5 parts ofN,N-dimethylaniline, followed by its stirring at 0°-5° C. for 2 hours.The resultant mixture was then poured in 500 parts of water, followed byits filtration. The resulting solid matter was washed with water andthen dried, thereby obtaining a crude product which was a compoundrepresented by the following formula: ##STR56##

The dichroic ratio of the above-purified dyestuff in "ZLI-1840" was 15.3at its maximum absorption wavelength of 540 nm.

EXAMPLE 12

Dispersed in 100 parts of chloroform ware 2 parts of a compound of thefollowing formula: ##STR57## followed by an addition of 0.9 part ofN,N'-dicyclohexylcarbodiimide. After stirring the resultant mixture atroom temperature for 2 hours, 20 parts of ethanol were added and theresulting mixture was stirred at 50° C. for 1 hour. The reaction mixturewas concentrated and n-hexane was added to the residue. The resultingmixture was filtered and the thus-obtained solid matter was dried,thereby obtaining a crude product which was a compound of the followingformula: ##STR58##

The dichroic ratio of the above-purified dyestuff in "ZLI-1840" was 16.4at its maximum absorption wavelength of 472 nm.

EXAMPLE 13

A compound represented by the following formula: ##STR59## wasdiazotized, followed by its coupling with α-naphthylamine to obtain acompound represented by the following formula: ##STR60## Five parts ofthe above compound and 3 parts of p-phenylbenzaldehyde were added to 200parts of toluene. The resultant mixture was heated under reflux for 5hours. During this heating, toluene was allowed to distill off graduallyso that the volume of the solution was reduced to about one half. Afterthe reaction, the solvent was distilled off under reduced pressure,followed by an addition of 150 parts of ethanol. The resultant mixturewas filtered, and the thus-collected solid matter was washed withethanol and then dried, thereby obtaining 5.6 parts of a compoundrepresented by the following formula: ##STR61##

The dichroic ratio of the above-purified dyestuff in "ZLI-1840" was 16.7at its maximum absorption wavelength of 521 nm.

EXAMPLE 14

A compound represented by the following formula: ##STR62## wasdiazotized in DMF, followed by its coupling with a compound representedby the following formula: ##STR63## to obtain a compound represented bythe following formula: ##STR64##

The dichroic ratio of the above-purified dyestuff in "ZLI-1840" was 19.6at its maximum absorption wavelength of 602 nm.

EXAMPLE 15

After adding 11 parts of a compound represented by the followingformula: ##STR65## in 150 parts of water, 120 parts of 35% hydrochloricacid were added. The resultant mixture was cooled below 5° C., followedby a dropwise addition of 40 parts of a 10% aqueous solution of sodiumnitrite. After stirring the mixture at the same temperature for 2 hours,sulfamic acid was added to obtain a diazotized liquid mixture. Afteradjusting the diazotized liquid mixture to pH=4, 50 parts of an aqueoussolution containing 6.2 parts of sodium α-naphthylaminomethanesulfonatewere added dropwise and the resultant mixture was stirred at 0°-5° C.for 2 hours. Thereafter, 1,000 parts of a dilute aqueous solution ofcaustic soda were added and the resultant mixture was stirred at 80° C.for 1 hour. After the reaction, the reaction mixture was neutralizedwith 35% hydrochloric acid and then filtered. The resulting solid matterwas washed with water and dried, thereby obtaining 17 parts of a crudeproduct which was a monoazo dyestuff represented by the followingformula: ##STR66##

In 100 parts of N,N-dimethylformamide, 4.2 parts of the above-obtainedmonoazo compound were dissolved, followed by an addition of 20 parts of35% hydrochloric acid. The resultant mixture was cooled below 5° C.,followed by a dropwise addition of a 10% aqueous solution of sodiumnitrite. After stirring the mixture at the same temperature for 2 hours,sulfamic acid was added to obtain a diazotized liquid mixture.

Added dropwise to the diazotized liquid mixture were a solution of 4.0parts of N,N-dimethylaniline dissolved in 10 parts ofN,N-dimethylformamide. The resultant mixture was stirred at 0°-5° C. for2 hours. After its adjustment to pH=4, 100 parts of ice water were addedand the resultant mixture was filtered. The thus-obtained solid matterwas washed with water and then dried, thereby obtaining 5.0 parts of adisazo dyestuff represented by the following formula: ##STR67## Thedisazo dyestuff was dissolved in toluene and then isolated and purifiedon a column chromatograph filled with silica gel, thereby obtaining thedisazo dyestuff.

In a small beaker, 0.1 part of the above disazo dyestuff was added to aliquid crystal mixture which consisted of 43% of4-n-pentyl-4'-cyanobiphenyl, 17% of n-propoxy-4'-cyanobiphenyl, 13% of4-n-pentoxy-4'-cyanobiphenyl, 17% of 4-n-octoxy-4'-cyanobiphenyl and 10%of 4-n-pentyl-4'-cyanoterphenyl. The resultant mixture was heated toabout 80° C. to convert it into a completely clear solution. Afterallowing the content to stand and to cool down, the resultant coloredliquid crystal solution was sealed in a liquid crystal display device.

The display device showed a purple color while no voltage was applied.When a voltage was applied, it became colorless only at its electrodesand very good contrast was produced. In addition, its dichroic ratio was13.8 at its maximum absorption wavelength of 545 nm.

EXAMPLE 16

In the same manner as in Example 15, the compound represented by thefollowing formula: ##STR68## was diazotized, followed by its coupling atpH=8-9 with phenol to obtain a compound represented by the followingformula: ##STR69## Then, 0.5 part of the above compound was dissolvedunder heat in 50 parts of N,N-dimethylformamide, followed by addition of2.0 parts of pyridine and 0.8 part of propionyl chloride. The resultantmixture was stirred at 80° C. for 3 hours. After cooling the reactionmixture down to room temperature, 100 parts of water were added. Thethus-obtained mixture was filtered and the resulting solid matter waswashed with water and then dried, thereby obtaining 0.5 parts of adisazo dyestuff represented by the following formula: ##STR70##

The dichroic ratio of the above-purified dyestuff in a liquid crystal"E-8" produced by BDH Corporation was found to be 11.4 at its maximumabsorption wavelength of 455 nm.

The structural formulae and dichroic ratios of further dichroic azodyestuffs of this invention will hereinafter be given in Table 1 throughTable 5.

                                      TABLE 1                                     __________________________________________________________________________                                                          Dichroic ratio/         Ex.                                                   dissolved color         No.                                                                              Structural formula                                 (E-8)                   __________________________________________________________________________    17                                                                                ##STR71##                                         9.0/yellow              18                                                                                ##STR72##                                         10.2/yellow             19                                                                                ##STR73##                                         12.4/bluish red         20                                                                                ##STR74##                                         11.0/bluish red         21                                                                                ##STR75##                                         9.8/yellow              22                                                                                ##STR76##                                         12.8/red                23                                                                                ##STR77##                                         15.1/blue               24                                                                                ##STR78##                                         12.5/red                25                                                                                ##STR79##                                         13.2/red                26                                                                                ##STR80##                                         12.9/red                27                                                                                ##STR81##                                         11.5/red                28                                                                                ##STR82##                                         12.1/red                29                                                                                ##STR83##                                         13.8/blue               30                                                                                ##STR84##                                         12.6/red                31                                                                                ##STR85##                                         12.8/red                32                                                                                ##STR86##                                         13.8/blue               33                                                                                ##STR87##                                         11.8/red                34                                                                                ##STR88##                                         13.1/blue               35                                                                                ##STR89##                                         10.4/red                36                                                                                ##STR90##                                         12.9/red                37                                                                                ##STR91##                                         13.6/reddish blue       38                                                                                ##STR92##                                         13.2/red                39                                                                                ##STR93##                                         13.5/reddish blue       40                                                                                ##STR94##                                         10.2/red                41                                                                                ##STR95##                                         12.5/red                42                                                                                ##STR96##                                         12.8/red                43                                                                                ##STR97##                                         13.1/reddish blue       44                                                                                ##STR98##                                         13.0/reddish blue       45                                                                                ##STR99##                                         12.0/red                46                                                                                ##STR100##                                        12.8/bluish red         47                                                                                ##STR101##                                        11.5/red                48                                                                                ##STR102##                                        11.3/blue               __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                                                           Dichroic ratio/        Ex.                                                    dissolved color        No.                                                                              Structural formula                                  (ZLI-1840)             __________________________________________________________________________    49                                                                                ##STR103##                                         14.2/blue              50                                                                                ##STR104##                                         13.6/blue              51                                                                                ##STR105##                                         16.2/purple            52                                                                                ##STR106##                                         13.5/orange            __________________________________________________________________________

    TABLE 3        Dichroic ratio/ Ex.  dissolved color No. Structural formula (ZLI-1565)          53      ##STR107##      17.9/bluish purple      54     ##STR108##      17.3/bluish purple      55     ##STR109##      16.8/bluish purple      56     ##STR110##      17.4/bluish purple      57     ##STR111##      15.1/purple      58     ##STR112##      14.7/purple      59     ##STR113##      14.3/purple      60     ##STR114##      14.6/purple      61     ##STR115##      15.5/purple      62     ##STR116##      14.3/purple      63     ##STR117##      13.8/purple      64     ##STR118##      13.0/purple      65     ##STR119##       9.9/orange      66     ##STR120##      11.5/orange      67     ##STR121##      11.2/orange      68     ##STR122##       9.9/orange      69     ##STR123##      12.0/orange      70     ##STR124##      10.5/orange      71     ##STR125##      13.5/purple      72     ##STR126##      11.5/orange      73     ##STR127##      11.4/purple      74     ##STR128##      10.8/orange      75     ##STR129##      19.1/bluish purple      76     ##STR130##      18.2/bluish purple      77     ##STR131##      17.6/bluish purple      78     ##STR132##      17.3/bluish purple      79     ##STR133##      17.3/bluish purple      80     ##STR134##      16.4/bluish purple      81     ##STR135##      17.6/bluish purple      82     ##STR136##      15.8/bluish purple      83     ##STR137##      18.0/purple      84     ##STR138##      17.4/purple      85     ##STR139##      15.8/purple      86     ##STR140##      14.6/purple      87     ##STR141##      15.2/purple      88     ##STR142##      15.7/purple      89     ##STR143##      15.4/purple      90     ##STR144##      15.9/purple      91     ##STR145##      14.4/yellow      92     ##STR146##      14.1/yellow      93     ##STR147##      16.3/purple

                                      TABLE 4                                     __________________________________________________________________________                                                          Dichroic ratio/         Ex.                                                   dissloved color         No. Structural formula                                (E-8)                   __________________________________________________________________________    94                                                                                 ##STR148##                                       15.2/yellow             95                                                                                 ##STR149##                                       16.9/purple             96                                                                                 ##STR150##                                       16.8/purple             97                                                                                 ##STR151##                                       12.2/red                98                                                                                 ##STR152##                                       12.6/red                99                                                                                 ##STR153##                                       15.2/purple             100                                                                                ##STR154##                                       15.6/red                101                                                                                ##STR155##                                       14.6/red                102                                                                                ##STR156##                                       17.1/blue               103                                                                                ##STR157##                                       16.9/blue               104                                                                                ##STR158##                                       13.3/yellow             105                                                                                ##STR159##                                       14.2/red                __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                                                     Dichroic ratio/              Ex.                                              max. absorption                                                               wavelength                   No.                                                                              Structural formula                            (ZLI-1840)                   __________________________________________________________________________    106                                                                               ##STR160##                                   14.6/452 nm                  107                                                                               ##STR161##                                   15.6/440 nm                  108                                                                               ##STR162##                                   13.6/441 nm                  109                                                                               ##STR163##                                   10.6/440 nm                  110                                                                               ##STR164##                                   14.2/542 nm                  111                                                                               ##STR165##                                   14.9/512 nm                  112                                                                               ##STR166##                                   16.3/572 nm                  113                                                                               ##STR167##                                   14.9/476 nm                  114                                                                               ##STR168##                                   18.9/561 nm                  115                                                                               ##STR169##                                   19.3/568 nm                  116                                                                               ##STR170##                                   14.8/441 m                   117                                                                               ##STR171##                                   18.5/532 nm                  118                                                                               ##STR172##                                   18.1/580 nm                  119                                                                               ##STR173##                                   17.2/452                     120                                                                               ##STR174##                                   17.5/516                     __________________________________________________________________________                                                     nm                       

We claim:
 1. A dichroic azo dyestuff represented by the followingformula: ##STR175## wherein A is --CN, --COOR₁, ##STR176## R₁ is alkyl,alkoxyalkyl, phenyl, cyclohexyl, or phenyl or cyclohexyl which aresubstituted with alkyl or alkoxy at the p-position thereof; R₂ and R₃are individually hydrogen alkyl, cycloalkyl, alkoxy, alkoxyalkoxy,cycloalkyloxyalkoxy, aryloxyalkoxy, aryl, halogen, cyano ordialkylamino; Y₁ and Y₂ are individually hydrogen, halogen, alkyl,alkoxy, cyano or hydroxyl; Ar₁ and Ar₂ are individually ##STR177## W₁and W₂ are individually --N═N-- or --N═CH--, but W₁ is --N═N-- when W₂is --N═CH--, m is 0 or 1, but m is 0 when W₁ is --N═CH--; and B ishydroxyl, alkoxy, alkoxyalkoxy, p-alkylbenzyloxy, ##STR178## but B ishydrogen, halogen, cyano, ##STR179## when W₁ or W₂ is --N═CN--, R₄ isalkyl, phenyl or cyclohexyl unsubstituted or substituted with alkyl oralkoxy at the p-position thereof, R₅ is hydrogen or alkyl, R₆ is alkylor ##STR180## R₇ and R₈ are individually hydrogen, halogen, hydroxyl,cyano, alkyl, dialkylamino, alkoxy, alkoxyalkoxy, ##STR181## and R₉ ishydrogen or alkyl.
 2. The dichroic azo dyestuff as claimed in claim 1,wherein A is --CN.
 3. The dichroic azo dyestuff as claimed in claim 1,wherein A is --COOR₁.
 4. The dichroic azo dyestuff as claimed in claim1, wherein A is ##STR182##
 5. The dichroic azo dyestuff as claimed inclaim 1, wherein A is ##STR183##
 6. The dichroic azo dyestuff as claimedin claim 1, wherein R₁ is --CH₃, --C₂ H₅, --(CH₂)₃ CH₃, --(CH₂)₇ CH₃,--C₂ H₄ CH(CH₃)CH₂ --(CH₃)₃, --(CH₂)₁₀ CH₃, ##STR184## --C₂ H₄ OC₂ H₅,or --C₂ H₄ O(CH₂)₃ CH₃.
 7. A dichroic azo dyestuff as claimed in claim 1or 2, wherein the carbon number of each alkyl group is 15 or smaller. 8.A dichroic azo dyestuff as claimed in claim 1, wherein R₂ and R₃ areindividually a hydrogen atom, --CH₃, --C₂ H₅, --(CH₂)₂ CH₃, --CH(CH₃)₂,--(CH₂)₇ CH₃, --CH₂ C(CH₃)₃, --(CH₂)₁₀ CH₃, --OCH₃, --OC₂ H₅, --O(CH₂)₇CH₃, --OC₂ H₄ CH(CH₃)CH₂ C(CH₃)₃, ##STR185##
 9. The dichroic azodyestuff as claimed in claim 1, wherein Y₁ and Y₂ are individually ahydrogen atom, --Cl, --Br, --I, --F, --CN, --OH, --OCH₃ or --CH₃. 10.The dichroic azo dyestuff as claimed in claim 1, wherein Y₁ and Y₂ areboth hydrogen atoms.
 11. The dichroic azo dyestuff as claimed in claim1, wherein Ar₁ and Ar₂ are individually ##STR186##
 12. The dichroic azodyestuff as claimed in claim 1, wherein B is a hydroxyl, --OCH₃, --OC₂H₅, --OC₄ H₉ (n), --OC(CH₃)₃, --O(CH₂)₅ CH₃, --O(CH₂)₇ CH₃, --OC₂ H₄CH(CH₃)CH₂ C(CH₃)₃, --O(CH₂)₁₂ CH₃, --OCH₂ CH₂ OC₂ H₅, --OCH₂ CH₂ OCH₂CH(CH₃)₂, ##STR187## --N(CH₃)₂ or --N(C₂ H₅)₂.
 13. The dichroic azodyestuff as claimed in claim 1, wherein when B is represented by##STR188## R₄ is --CH₃, --C₂ H₅, --(CH₂)₃ CH₃, --(CH₂)₇ CH₃, --C₂ H₄CH(CH₃)CH₂ --(CH₃)₃, --(CH₂)₁₀ CH₃, ##STR189##
 14. The dichroic azodyestuff as claimed in claim 1, wherein when B is represented by##STR190## R⁵ is a hydrogen atom, --CH₃, --C₂ H₅ or --C₄ H₉.
 15. Thedichroic azo dyestuff as claimed in claim 1, wherein when B isrepresented by ##STR191## R⁶ is --CH₃, --C₂ H₅, --C₄ H₉ (n), --CH₂CH(CH₃)₂, ##STR192##
 16. A liquid crystal composition containing, in aform dissolved in a liquid crystal, at least one of dichroic azodyestuffs represented by the following formula: ##STR193## wherein A is--CN, --COOR₁, ##STR194## R₁ is alkyl, alkoxyalkyl, phenyl, cyclohexyl,or phenyl or cyclohexyl which are substituted with alkyl or alkoxy atthe p-position thereof; R₂ and R₃ are individually hydrogen, alkyl,cycloalkyl, alkoxy, alkoxyalkoxy, cycloalkyloxyalkoxy, aryloxyalkoxy,aryl, halogen, cyano or dialkylamino; Y₁ and Y₂ denote individuallyhydrogen; halogen, alkyl, alkoxy, cyano or hydroxyl; Ar₁ and Ar₂ areindividually ##STR195## W₁ and W₂ are individually --N═N-- or --N═CH--,but W₁ is --N═N-- when W₂ is --N═CH--, m is 0 or 1, but m is 0 when W₁is --N═CH--; and B is hydroxyl, alkoxy, alkoxyalkoxy, p-alkylbenzyloxy,##STR196## but B is hydrogen, halogen, cyano, ##STR197## when W₁ or W₂is --N═CN--, R₄ is alkyl, phenyl or cyclohexyl unsubstituted orsubstituted with alkyl or alkoxy at the p-position thereof, R₅ ishydrogen or alkyl, R₆ is alkyl or ##STR198## R₇ and R₈ are individuallyhydrogen, halogen, hydroxyl, cyano, alkyl, dialkylamino, alkoxy,alkoxyalkoxy, ##STR199## and R₉ is hydrogen or alkyl.
 17. Thecomposition as claimed in claim 16, wherein the concentration of thedichroic azo dyestuff in the liquid crystal composition is aconcentration of 0.01-10 wt.%.